Magnetic resonance imaging (MRI) is an enabling technology platform in biomedical imaging that offers a wide variety of diagnoses including investigation of tumor sizes, detection of restenosis of vessels or stent lumen, imaging of neural signals (fMRI) and the observation of tissue and tissue regeneration during the making of and after incisions. MRI has evolved into a major imaging tool in medicine due to its non-invasive and safe nature.
The MR imaging technique exploits the interaction of applied magnetic fields with nuclei in the target area of the material of interest (such as a portion of a patient's body), and the detection of signals resulting from that interaction. In the technique, a strong static applied magnetic field splits energy levels of the nuclei of interest based on the interaction of the spin of the nuclei and the static field, and a time-varying magnetic field is tuned to the resonance frequency corresponding to the split energy levels. Resonance transitions, which indicate the presence of nuclei of interest, cause a change in magnetic field, which may be detected by receiver coils and processed into an image depicting the positional distribution of the nuclei of interest.
In medical imaging, the hydrogen atom (H), which is abundant in all organic tissue, is the nucleus of interest, and hence the properties of H have the key role in determining the operating MR frequency. Hydrogen atoms, which are aligned in the direction of the static magnetic field, are excited with a radio frequency (RF) field to absorb incident energy. This energy is re-emitted to the environment in accordance with relaxation times that depend on the type of the tissue containing the excited H nuclei of interest. The receiver coils detect the resulting signals, which are processed into an image.
The relationship between the resonance frequency (fres) of H and the magnetic flux density (B) is given by Equation 1.
                              f          res                =                              y                          2              ⁢                                                          ⁢              π                                ⁢          B                                    Equation        ⁢                                  ⁢        1            
Where, Equation 1′, γ is the gyromagnetic ratio of H and has a value of 42.575 MHz/Tesla. Commercially available MRI scanners for use with humans apply static magnetic fields in a range of approximately 0.5 to 7 Tesla, which correspond to resonance frequencies in H that vary between approximately 21.2 to 300 MHz, respectively. The most widely used systems for medical imaging apply static magnetic fields in the range of approximately 1.5 to 3 Tesla, which corresponds to resonance frequencies in the range of approximately 64 to 127 MHz, respectively.
For some tissue, due to the lack of hydrogen atoms in the target area, or small volume of the target area, it is difficult to obtain a good MRI signal to construct a meaningful image. Additionally, in cases where an implant has been inserted into the patient, obtaining MR images of the area of the body near the implant may be of interest. However, in many cases, the presence of the implant renders that area opaque to magnetic fields, leading to small or poor MRI signals that are not adequate for the desired imaging.
One way of trying to overcome such problems involves the transmission of higher power for absorption by the tissue of interest (e.g., by increasing the intensity of the applied time-varying magnetic field), potentially leading to higher amplitude signal emission from the tissue. However, increasing the absorption of power by tissue may cause undesired and potentially harmful heating of the tissue. In certain cases, such heating may damage the tissue irreversibly.
Another approach involves, as an alternative to increasing transmitted power to the tissue of interest, focusing and rendering more intense the applied time-varying magnetic field at only the points of interest or in an area that is relatively localized around the points of interest. An electrical resonator whose resonance frequency matches the frequency of the time-varying magnetic field that is applied and that is located at or near the points of interest may enhance localization of the time-varying magnetic field in its vicinity, and lead to the emission of a stronger or higher-quality signal from the points of interest. Such a resonator couples the emitted signal to the receiver coil without necessitating a wired connection.
As depicted in FIG. 1, a resonator (110) may comprise an electrical circuit that includes ideal lumped circuit elements, such as an inductor (L), capacitors (C and Cgap), and a resistor (R). In the embodiment of FIG. 1, each of the inductor and capacitors are separate elements (i.e., lumped elements).
FIG. 2 depicts a known physical layout of a resonator embodiment (210), which may be modeled by the ideal circuit depicted in FIG. 1. Resonator 210 (which is also called a “split-ring resonator”) includes semi-ring (220) that is formed from a metallic or conductive, semi-circular element, and gap (230), which is an interruption of the conductive path formed by semi-ring (220), and which may comprise either empty space or a dielectric material. A time-varying magnetic field that is applied to semi-ring (220) induces a time-varying current in semi-ring (220) in accordance with Faraday's law; thus semi-ring (220) may be modeled as an inductor (L). Additionally, the metal or conductor forming semi-ring (220) may have a resistance (R). Furthermore, the presence of gap (230) (including any dielectric material formed therein) may cause charge build-up on the parts of semi-ring (220) adjacent to gap (230) when an electrical current is applied or is present in semi-ring (220). For that reason, gap (230) (including any dielectric material formed therein) and the relevant adjacent parts of semi-ring (220) may be modeled as a capacitor (C). Thus, resonator (210) effectively forms a resonant RLC circuit. As is known to those of ordinary skill in the art, the resonant frequency of such an RLC circuit is given by Equation 12.
                              f          resRLC                =                  1                      2            ⁢                                                  ⁢            π            ⁢                                                            L                  etkin                                ⁢                                  C                  etkin                                                                                        Equation        ⁢                                  ⁢        2            
A time-varying current induced in resonator (210) in turn generates a time-varying magnetic field in accordance with the displacement current term in Ampere's law and the right-hand rule. The magnetic field that is generated is in a direction that is normal to the plane containing resonator (210), with the exact direction (i.e., whether the direction is into the plane or out of the plane containing resonator (210) along the normal direction) of the magnetic field depending on the direction of the induced current in resonator (210) in accordance with the right-hand rule.
As is known to those of ordinary skill in the art, a parameter that characterizes a resonant LCR circuit is its quality factor (Q-factor) that is given in Equation 3.
                    Q        =                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                                          f                resRLC                            ·              L                                R                                    Equation        ⁢                                  ⁢        3            
The Q-factor provides an easily calculable measure of the strength of a resonance of a resonator, with high values of Q corresponding to a highly resonant circuit in which the resonance is observed over a narrow bandwidth of frequencies.
If the resonance frequency (fresRLC) of resonator (110) is the same or approximately the same as the frequency of the applied time-varying magnetic field of the MRI device, resonator (110) will effectively amplify the total time-varying magnetic field (compared to the situation in which resonator (110) is not present) in the vicinity of resonator (110). This, in turn, will cause an enhanced MR signal to be received from those portions of the target area for imaging that are in the vicinity of resonator (110).
There have been previous efforts to utilize a resonator to enhance the signal generated during a MRI measurement. In particular, resonator structures have been proposed for MRI signal and resolution enhancement that include a lumped capacitor. For example, U.S. patent publication nos. U.S. 2010/0127707 and U.S. 2010/0033178 disclose various embodiments of a split-ring resonator (SRR), similar to the embodiment discussed above in connection with FIG. 2, in which the capacitor is a lumped circuit element that is formed from the end of the SRR, which faces each other and the gap (230) separates the two ends. In one embodiment, U.S. 2010/0127707 discloses a resonator comprising two square-shaped split-ring resonators (where the two square-shaped split-ring resonators are not in conductive contact with one another) that are oriented parallel to one another and axially aligned, with a dielectric layer sandwiched between the two square-shaped split-ring resonators.
Another lumped-capacitor approach has been suggested in the context of a resonator that is part of or attached to the scaffold part of a stent for implantation in the body of a human. For example U.S. 2007/0239256 describes a stent structure with a sheath that includes two distinct coils with a capacitor placed between the two coils. Similar or analogous embodiments that include lumped capacitors are disclosed in, for example, U.S. Pat. Nos. 6,767,360; 7,279,664; 7,304,277; 7,335,229; 7,423,496; 7,595,469; 7,766,958; 7,778,684; 7,812,290; 7,838,806; 7,988,719; 8,046,048; 8,058,593; 8,066,759 and applications U.S. 2010/0286764; U.S. 2008/012854; U.S. 2007/0062933; U.S. 2007/0032862; U.S. 2004/0254632.
Lumped capacitors in MRI-enhancement resonators are too bulky and mostly not biocompatible, and therefore not suitable for implantation into the body. Generally, the capacitance values achievable through a lumped capacitor are not sufficiently high to yield resonant frequencies for the corresponding resonator that are sufficiently low for MRI applications, when the dimensions of the resonator are sufficiently small for purposes of implantation into the human body. Such structures cannot be implanted in the body for medical purposes, and usually may only be used as external surface MRI enhancement resonators. See M. J. Freire et al., “Experimental demonstration of a μ=−1 metamaterial lens for magnetic resonance imaging; Applied Physics Letters, 93, 231108 (2008).”
Another disadvantage of a resonator that includes a lumped capacitor is that it usually causes imaging artifacts such as black spots in the resulting MR image, due to the fact that the electric field of the capacitor leaks outside the volume enclosed by the capacitor.
Flexible and biocompatible resonators with wired connections to the detector device have been proposed that use capacitors with thin film dielectrics. Such resonators may in principle be implanted into the body. However, such devices are also not sufficiently small, and the need for a wired connection prevents or complicates applications that envision implantation of the device into the human body. See, e.g., R. R. A. Syms et al., “Thin Film Detector Systems for Internal Magnetic Resonance Imaging”, Sensors and Actuators A 163, 15-24, (2010).
Thus, there is a need for a resonator for MRI enhancement that is sufficiently small to be used in in-vivo applications, that does not impair the resulting MRI image, and that provides effective MR image or resolution enhancement. Such a resonator should be biocompatible, compact and small, not wired, and preferably, flexible.